The present invention relates generally to methods used to determine magnetic properties of materials. In particular, this invention relates to systems that determine the grain size distribution, σV, and anisotropy field distribution, σHK, in thin film granular or nanoparticle material.
In order to achieve magnetic recording in advanced perpendicular media beyond 100 Gbits/in2, it is necessary to have media with ultra-high anisotropy constant, KU, and very narrow anisotropy field distribution, σHK. Increases in areal density of the recording media have relied on a reduction of the magnetic grain size, V (where V is volume), and the grain size distribution, σV to keep media noise within acceptable levels while at the same time maintaining thermal stability. To achieve this goal, the media must be carefully designed and engineered to reduce both the grain size, V, and anisotropy field distribution, σHK.
In order to increase the areal density of a disc it is necessary to create more bits on the surface area of the disc. Each bit consists of a plurality of grains, on the order of 10 to 100, that hold the magnetic state of each bit. Presently, grains are on the order of 10 to 20 nanometers in diameter, with advances demanding much smaller diameters. In order to increase areal density, it is necessary to either use fewer grains per bit or to decrease the size of the grains. It is undesirable to use fewer grains per bit because this increases the error and noise associated with each bit as there are fewer grains left to hold the magnetic orientation of the bit. Thus, it is necessary to decrease the size of the grains in each bit to increase areal density.
Decreasing the grain size has limitations. The switching energy of a grain is the energy required to switch the magnetic orientation of the grain along the easy axis.EB=KUV(1−HDC/HK)2  Equation [1]The energy barrier, EB, is a function of an anisotropy energy KU (which is the energy difference between the easy axis and hard axis orientation of the grain), grain volume V, the DC field strength HDC, and anisotropy field strength HK. Each grain also has an associated thermal energy, kBT, that is the temperature dependent thermal energy of the grain, where kB is Boltzmann's constant and T is the absolute temperature. It is desirable that this only happen during write operations by the writer. Since the energy barrier is a function of grain volume V, with small grains the thermal energy can be greater than the energy barrier. This can result in undesired switching of the grain. Thus, the grains of magnetic media should not be so small that the thermal energy of the grain is greater than the energy barrier. In other words, the anisotropy energy KUV of each grain must remain higher than the thermal energy KBT.
The grains comprising the magnetic media are not uniform in size. They are distributed over a range of grain volumes (or grain diameters). Ideally, all grains would be of the same size in order to ensure each grain is at the optimal volume to overcome thermal switching and maximize areal density. This, however, is not possible. Thus, it is necessary to have as narrow a distribution of grain volume (or diameter) as possible near the optimal grain volume (or diameter) in order to eliminate small grains that can be switched by thermal energy. Similarly, it is necessary to have a narrow KU distribution in order to insure that the grains are properly switching in a narrow range during the writing process by the writer, such that a narrow transition width will be achieved to support high recording density.
Media noise is dependent on grain size. Media noise is a result of “saw-toothed” transitions between bits on magnetic media. This means that one bit's region on the disc track is not clearly distinct from the next bit's region. These irregular boundaries result in noise as the reader transitions from one bit to the next. A low noise magnetic medium is accomplished by having small grain size. Smaller grain size helps average the arbitrary nature of the grains over more grains per bit, thus providing a smoother transition between bits. Decoupled grains also help to reduce media noise because the decoupled grains provide a more distinct transition between bits as clear grain boundaries are present in the micro-structure between uncoupled grains.
Direct measurement of grain size distribution σV (or σD, where D is grain diameter) and anisotropy field distribution σHK is crucial, as it enables key experimental insights into the main factors controlling these distributions and allows development of experimental materials and processing strategies to reduce them. In particular, as advances in magnetic recording move toward the use of thin film granular material, σV and σHK become even more important. Accurate determination of σV and σHK, however, remains a challenge, especially in thin film decoupled granular material.
In thin film granular material, determination of grain size distribution and anisotropy field distribution is difficult. Magnetic measurements, such as susceptibility, are difficult in granular material since the magnetic signal generated by these grains is low. Also, thin films produce weaker signals because there is less material to produce a change in signal. Therefore, highly sensitive methods are needed.
Previously, both easy-axis and hard-axis hysteresis loops have been adopted to extract σHK. Papusoi et al., “Magnetic Anisotropy Field Dispersion Characterization of Advanced Perpendicular and Longitudinal Media,” IEEE Transactions On Magnetics, Vol. 38, No. 4, July 2002, pp. 1687–1692. However, the extracted σHK is convoluted with the crystalline anisotropy angular dispersion and the grain size dispersion.
Other techniques have used the temperature dependence of the complex transverse AC susceptibility (T. Jonsson, et. Al. J. Magn. Magn. Mater., 168, 269 (1997)) or have used a Micro-SQUID noise technique (S. I Woods, et. al. Phys. Rev. Lett., 87, 137205 (2001)) to measure energy barrier distributions in superparamagnetic nanoparticles. However, since both of these methods rely on measurements in the superparamagnetic state, they are of limited value in assessing dipsersions in thermally stable ferromagnetic media. This is because at temperatures near or above the Curie temperature needed to induce sufficient thermal fluctuations, the magnetic properties, spontaneous magnetization and anisotropy, and their dispersions are significantly different from respective values at the temperature of interest. Additionally, these methods do not allow for grain size distribution measurements.
In a recent paper in the theoretical field of susceptibility, which is herein incorporated by reference, Papusoi mathematically derive the relationship between the complex transverse AC Susceptibility with anisotropy field distribution, σHK, and grain size distribution, σV. Papusoi, “The Complex Transverse Susceptibility,” Physics Letters A, Vol. 265, Issues 5–6, February 2000, pp. 391–402. Papusoi teaches that the measured complex transverse AC susceptibility contains information on both σV and σHK. However, the separation of σHK from σV in the complex transverse AC susceptibility was not shown. Papusoi's calculations depend on the thermal relaxation rate of individual grains, as relied on in the above mentioned methods. Papusoi teaches that the thermal relaxation can be controlled by applying a DC field. Thus, susceptibility calculations can be carried out at the temperature of interest without altering the magnetic properties of the materials, namely spontaneous magnetization and anisotropy.
Papusoi suggests a pick-up coil detection scheme for measuring complex transverse AC susceptibility in recording tape. This method, however, is not accurate enough for use on thin film decoupled magnetic material. Specifically, the pick-up coil method suggested by Papusoi creates too much noise in the system at low levels to detect the small changes created by the reorientation of the individual grains. The pick-up coil method requires applying an AC field with a coil and a DC field to the magnetic sample material. The AC coil is present within the DC field, which causes the AC coil to vibrate. Thus, the detecting pick-up coil also vibrates when coupled with the AC coil. This causes noise greater than the susceptibility measurement in thin film granular material.
The pick-up coil method suggested by Papusoi also has another drawback when associated with measuring in magnetic recording media with a thin film top layer and a soft underlayer. The experiment Papusoi suggests uses a sample with a thickness of 200 nanometers. The area of interest in thin film magnetic recording media is the ultra-thin top and seed layers that have a thickness of ˜20 nanometers. The pick-up coil method suggested by Papusoi will measure beyond the top and seed layers. The susceptibility measurement will then be an average of the whole sample, not just the top layers. Thus, the pick-up coil method is not able to obtain precise information from magnetic disc media with ultra-thin film top layers.
Thus, there is a need in the magnetic recording industry to accurately determine complex transverse susceptibility in ultra-thin film decoupled granular material so that σV and σHK can be accurately derived and de-convoluted.